The overall objective is to investigate the molecular mechanisms and genetic control of cellular repair of damaged DNA and to determine the effect of cellular repair mechanisms on genetic recombination, DNA replication and mutation induction. These problems are being studied in a simple eucaryotic organism, the yeast Saccharomyces cerevisiae, where both mutants with repair deficiencies exist and a well-defined mutational system is available. Our approach is to use biochemical, molecular biological and genetic means to achieve these objectives. Mutants in the nine genes involved in excision of UV-induced pyrimidine dimers are being analyzed to determine which are defective in excision and which in incision. These experiments are being carried out in rad cdc9 double mutants since cdc9 is temperature-sensitive for ligase activity, which is required in the final step of excision repair. Experiments dealing with the nature of "post-replication repair" of UV damage in excision defective mutants are being carried out. We hope to characterize this recovery process in yeast and to determine whether an inducible component exists. Studies on characterization of repair enzymes have begun with a 2000-fold purification of uracil DNA glycosylase of yeast.